Electric discharge device and control apparatus therefor



E. D. MCATHUR ELECTRIC DISCHARGE DEVICE AND CONTROL APPARATUS THEREFOR Fig. If f Filed April 28, 1934 ANODE POTENTIAL VOLTS 2 Sheets-Sheet 1 Fig. 3.

FigA u a l0 5'0 FIELD STRENGTH- GAUSG Inventor: Elmer D. McArthur,

His Attorney.

E. D. M ARTHUR ELECTRIC DISCHARGE DEVICE AND CONTROL APPARATUS THEREFOR Filed April 28, 1954 Fig. 5.

2 Sheets-Sheet 2 /0 20 30 40 F/ELD STRENGTH 6/1086 2 4- 6 8 /0 l2 l4 /6 I6 20 2224262850 Inventor Elm er D. McArtur,

His Attorney.

atented Apr. 28, 1936 pr OFFICE ELECTRIC DISCHARGE DEVHIIE TROL APPARATUS TEE]? R Ehncr l). McArthur, Schenectady, N Y assignor to General Electric Company, a aeration of New York Application April 2d, 1934, Serial lilo. 722,918 ii Claims. (cram-2v) The present invention relates to electric discharge apparatus and more particularly to thermionic devices containing a cathode and an anode and having a discharge of an arc or glow form passing between the electrodes. In devices of this sort, an lonizable medium is generally employed, such as mercury vapor or inert gas, the purpose of which during operation is to neutralize space charge and to permit cumulative ionization.

' Tubes of this character are capable of handling relatively large amounts of current, since the voltage losses are reduced to a minimum.

in order to control this large amount of power, it is usual to employ an electrostatic control member or grid positioned between the cathode and anode. The function of the grid is to control the initiation of the arc and when alternating voltage is impressed between the cathode and anode, the grid ordinarily triggers the tube at the same point in each positive half-cycle of the voltage in order that the average current over succeeding periods of time may be constant. However, when electrostatic control members are employed in tubes of this character, particularly those having unusual electrode design or when operating under exaggerated conditions, certain dimculties may attend the use of such a-memher. For example, the grid may become an emitter of electrons, or when voltage transients are set up in the circuits associated with the tube, these transients may be transferred by inductive electrostatic efiect to the grid and thereby reduce its control function. In order to eliminate some of these dimculties, it has been proposed to move the control member, from within the tube to a position on the exterior there-oi but in this case, other diculties have been found which limit to a certain extent the field of usefulness of such devices.

in accordance with the present invention, 1 provide an entirely novel form of control in devices of this character, a control which does not require the usual form of control electrode, i. e. an electrostatic control member, either within or without the device, and yet one which is practical and efiective.

An object of my invention, therefore, is to provide a novel form of control for tubes of the are or glow type, one which eliminates the necessity for the usual electrostatic control member, and yet, provides a powerful control chest for accurately determining when the arc shall start during the recurring positive half-cycles of the applied anode alternating voltage. it further object is to simplify the construction and design of controlled are or glow discharge devices. In carlying out these objects, the invention contemplates the use of a magnetic field which is so positioned with respect to the tube as to affect the direction of movement of theelectrons which a normally pass to the anodeand which cause initiation of the are or glow. The efiect of the magnetic field is to reduce but preferably entirely to prevent ionizing collisions between the electrons emitted by the cathode and gas or vapor to molecules of the ionizable medium within the envelope.

The invention will be better understood when reference is made to the following description and accompanying drawings in which Fig. l is a it View, partly in diagram and partly in section, of an arc discharge tube and an associated magnet system for controlling initiation of the arc; Fig. 2 is a diagram showing the equipotential lines in the tube illustrated in Fig. 1; Figs. 3 and i are diagrams oi certain control characteristics of the apparatus shown in Fig. i; Fig. 5 illustrates a modified form of tube employed with a magnetic field similar to that shown in Fig. 1; while Figs. 6 and l depict certain control characteristics of the 26 apparatus illustrated in Fig. 5.

Referring more icularly to Figs. 1 and 2, numeral l designates an envelope of vitreous material which is provided at the top, as shown, with a metal terminal member 2 and at the bottom, 30 with a metal base It carrying a plurality of terminal pins insulated from one another. The envelope is provided on the interior with a reentrant stem and press 5 and contains a cathode h, an anode l and an electrode t, the purpose of which will be explained hereinafter. The cathode may be of the conventional indirectly heated type and consist of an outer cylindrical member and an inner cylinder 9 secured to the outer cylinder by vanes ill. The recesses formed between the cylinders and the vanes may be coated with electronically active material. The inner cylinder contains a centrally positioned heater ll secured at one end to the cylinder 9. A cathode of this type is shown and claimed in the Hull and Ruggles Patent No, 1,924,318, granted August 29, 1933.

The cathode is preferably supported from the press by a pair of rigid leading-in conductors it which carry the load current, also the current to the heater ill. The return conductor for the heater current is constituted of a leading-in wire it sealed in the press.

The anode l is mounted preferably at the opposite endoi the envelope from the cathode and may be constituted of an inverted dish-shaped metal member secured in place by the equidistantly spaced wires l4 and the central wire l5 which carries a pant-leg I6. The wire I 5 is extended through the glass envelope to connect with the terminal 2.

The cylindrical member 8 which surrounds the anode and cathode is provided with a. transverse metal web I8 having a central opening IS. The cylinder is mounted in place by means of a plurality of equidistantly spaced rods 20 secured to a screw clamp member 2| which surrounds the reentrant stem. Electrical connections are taken from the electrode 8, or rather, from the clamp 2|, to one of the contact pins 4. There are also connections between the cathode 6 and one of the pins 4, and between the heater H and another of the pins.

The envelope contains an ionizable medium which may consist of inert gas, such as argon, at a pressure of from 50'to 800 microns, or vapor, such as caesium, or mercury.

Alternating voltage may be aplied between the anode l and the cathode 6 by means of an alternator 22 through an adjustable step-up transformer 23. The heater H may be supplied with energy from a battery 24. It is sometimes desirable to maintain the electrode 8 at a different potential than the cathode and as shown, there is provided a battery 25 and a potentiometer 26, the battery being preferably so poled as to apply a negative charge on the electrode, the voltage of which may be indicated by a meter 21. When a tube and system, such as described, are operated, an are or glow is formed between the oathode and anode during each recurring positive half-cycle of the anode voltage, provided the negative bias on the electrode 8 is not too large.

In accordance with the present invention, I

propose to control the average amount of current flowing through the tube under these conditions, which control cooperates with the normally biased electrode 8 to determine precisely the point in each positive half-cycle of anode voltage at which the arc will start." For this purpose, there is provided a magnetic field which intercepts the space between the upper end of the cathode 6 and the transverse member or partition [8 of the electrode 8, the field being so directed as to pass through the tube in a direction normal to its longitudinal axis. The magnetic field may be obtained from a. permanent magnet, but I prefer to employ an electromagnet which may consist of a U-shaped laminated core 29, with pole pieces directed toward the tube as shown, and having wound on one of its legs, a coil 3| in series with a battery 32 which may be poled in either direction as indicated, and a variable resistance 33.

When a magnetic field obtained in this manner is directed across the tube, the point in each positive half-cycle of anode voltage at which the tube is triggered may be positively controlled by the magnetic field, hence by the current flowing through the coil 3|. In general, the stronger the field, the longer will the arcbe restrained in each positive half-cycle and the later in the halfcycle will the are be initiated.

While I do not wish to be limited to any theory, the operation of the tube under these conditions appears to depend upon the deflection of the electrons emitted by the cathode out of their normally straight path which would take them through the aperture l9 to the anode 1. There is an electric field extending in substantially straight lines between these electrodes due to the voltage applied by the alternator 20. It is well known that electrons moving in an electric field which is normal to a magnetic field do not follow the direction of the electric field. The average direction of the motion of the electron is inclined away from the direction of the electric field by an angle 0 which may be calculated from the following equation:

HeX

tan -m where H is the magnetic field density,

6 is the charge of the electron,

M is the mass of the electron,

V is the velocity of the electron,

A is the mean free path of the electron among the gas molecules.

This relation shows that for a constant gas pressure, the field H required to produce any desired angular deflection depends on the velocity of the electrons. The less the velocity of electrons, the less will be the necessary field to produce the required angular deflection. By referring to the diagram in Fig. 2 which shows the electrical conditions in terms of equipotential lines, obtaining in a tube immediately prior to the initiation of an arc discharge, it will be noted that the volume bounded by the transverse member l8, cylindrical member 8 and the cathode 6 is an extremely low potential region. The potential distribution lines indicate that practically all of the voltage drop in the tube is confined to the space between the transverse member l8 and anode i. It will of course be understood that the the potential distribution lines are not absolutely acurate but only relatively so, because the plot does not include the effects principally due to positive ion space charges in the region between the anode and transverse member 18. Electrons cannot attain high velocities in the space between the cathode and member I8 and their paths, therefore, can be deflected through large angles with relatively weak magnetic fields. This figure also shows that electrons which normally would go through the opening in the baffle l8 to the anode can be deflected by a transverse magnetic field into a region where the electric and magnetic fields will return them to the oathode. When the potential of the electrode 8 and its bafile is only slightly negative or a few volts positive with respect to the cathode, many defiected electrons are probably collected by the electrode and to that extent, the latter serves as a barrier to the electrons. If, however, the electrode 8 is made several volts negative, the electrons will return directly to the cathode. In either case, the electron current flowing into the region between the anode and baifie i8 is reduced and the breakdown voltage increased. The term breakdown voltage is intended to mean the amount of anode voltage necessary to cause initiation of an are or glow discharge for a given set of electrical conditions within the tube.

Consequently, when magnetomotive force is produced in the core 29 by the coil 3|, the magnetic field, which preferably intercepts the space between the upper end of the cathode 6 and the baflie I8, tends to deflect the electrons which may either be collected by the electrode 8 or returned to the cathode, depending upon the polarity and magnitude of the bias given to the electrode. This bias of course may be delicately controlled by the potentiometer 26. By varying the amount of resistance I! in the coil circuit and the amount and polarity oi the bias; it is apparent that the magnetic field cooperates with the deflecting chamber contained within the electrode 8 to determine how many oi the electrons pass through the opening I! to reach the anode. The smaller the number of electrons which get through the opening IS, the less is the likelihood o1 producing cumulative ionization within the device.

It will be noted that the cylindrical portion of the electrode 8 serves as a shield to protect the discharge path from electrical charges on the interior of the envelope. If this shielding were not present, electrons leaving the cathode might still be deflected toward the walls of the envelope but they would be reflected by the negative charge built up on the glass. These electrons would then follow along the walls toward the anode and eventually reach the latter. This condition oi course would detract from the control efiects of the magnetic field operating in coniunction with the deflection chamber so that in order to raise the breakdown voltage or the tube, it would be necessary to increase the strength oi the magnetic field. v

The general requirements ci a tube design in which a magnetic field may be satisfactorily employed for controlling the initiation'oi an are or glow discharge are:

i. A relatively large volume for a deflecting chamber.

2. Bumcient shielding so that the electric held in the chamber is low.

3. Conducting walls tor the chamber.

t. A chamber partially bounded by the oath ode.

The term deflecting chamber, as reierred to in the requirements noted immediately above, is intended to mean the space contained between the upper edge or the cathode and the lower surface oi the transverse member or bade it and bounded circumierentially by the cylindrical electrode b.

When the above conditions are met in a substantlal degree, it has been iound that practically all tubes oi the electrostatically controlled arc discharge type which have heretofore been provided with an electrostatic control or grid member, can be successfully controlled by relatively weah transverse fields, provided the electrode within the tube which provides the dedection chamber is given the proper bias, depending upon the shape, size and position oi the electrode with respect to the other electrodes.

ltlgs. 3 and t show certain operating characterlstics oi the typical tube and magnetic field shown in Fig. l. in Fig. 3, curves are plotted i'or diflerent strengths of magnetic field immediately preceding the breakdown of a tube or the initiation of an arc, with anode potential in volts as ordinate, and the potential of the electrode t in volts as abscissa. Taking a typical curve, i'or example, the no gauss curve, it will be noted. that when a potential of approximately -3 volts is applied to the electrode t, the manimum anode voltage which may be applied to the tube before breakdown, is approximately 275 volts. When the magnetic field is increased to approximatelydil gausses as shown on the uppermost curve in this figure, and assuming the same potential on the electrode 3 (3 volts), the breakdown anode voltage is raised to 700.

Fig. 4 shows the effect of changes in the temperature oi. the condensate in a tube employing mercury vapor on the field strength and anode potential to cause breakdown of the tube. Taking an ordinary condensate temperature of approximately 40" C. and assuming that. a maximum potential or 650 volts is applied to the anode, a magnetic field or approximately volts will prevent a tube having the characteristics shown in this figure from breaking down and causing initiation oi an are or glow discharge through the mercury vapor.

Fig. 5 shows a construction oi tube which is designed to operate more efficiently in con nection with a transverse magnetic held than the tube shown in Fig. 1. It is apparent that in the tube illustrated in Fig. i, there is considerable magnetic reluctance or resistance oflercd by the spaces contained between the cylinder t and each pole piece, and this magnetic resistance represents a loss which must be compensated for by a stronger magnetic held. Consequently, in the tube shown in Fig. 5, there is provided a pair of vanes it, or magnetic material, which extend from the inner surface of the envelope, adjacent the position oi the pole pieces to a position which abuts the cathode cylinder to. The cylinder at is oi longer length than the corresponding cylinder t shown in Fig. 1, because it has been extended beyond the interior vanes bl which are indicated by the dotted lines, to term an electron-deflecting chamber oi relatlvely large volume at the upper end of the cathode. lithe vanes 35 are provided with arouate flanges at their outer ends which iit snugly against the inner wall oi-the envelope, and at their inner ends are secured directly to the cylinder it.

The vanes it may help to support a cylindrical member ti, open at the top but closed at the bottom by a transverse member or bae it which is provided with a central opening it. The cylinder bi extends upwardly, preferably as tar as or beyond the rear surface oi the anode it. The anode may be mounted within the envelope in a similar manner to that described in connection with Fig. l. The cylinder tilt is also supported truth the combined stem and press it oi the tube by means oi several equidistantly spaced rodsll which terminate at their lower ends in a screw clamp it, which fits tightly over the stem.

The cathode. it which preferably is oi the indirectly heated type, similar to that described in connection with Figs. 1 and 2, may be heated by a centrally positioned heater ii energised by a battery til.

The envelope may contain mercury or other vapor, or inert gas, at a pressure suincient to support an arc-like or glow discharge at the impressed voltages The magnetic field is provided by a core it, magnetically energized by a coil ill, a battery it and a variable resistance ti. Voltage may be agoplied between the anode db and the cathode it by means of an alternator ht through an adjustable step-up transformer hi. it will be noted that there is no biasing battery and potentiometer similar to that shown in Fig. l for the reason that the cylinder bi is metallically connected to the cathode. hence is maintained at cathode potential.

As in the case oi the apparatus shown in Fig. l, the tube shown in Fig. 5 lends itself readily to the control by magnetic effects of the initiation of the arc or glow discharge. Thus the relation between the voltage necessary to cause breakdown and the strength of the magnetic field which is opposing the efiect of the breakdown voltage is depicted in Fig. 6 by a plot drawn against plate voltage at breakdown as ordinate and field density in gausses as abscissa. When a field strength 01' approximately 26 .gausses,- for example, is applied to the deflection chamber contained within the upper end of the cylinder 36, it takes an anode voltage of approximately 560 to break the tube down. Lower intensities of field strength permit the are or glow to be initiated by lower anode voltages.

In Fig. 'l, the relation between the average current output of a tube of this type and the field strength is shown. It will be noted that a positive control of this current is effected by a relatively weak magnetic field between current values of from 1 to 2 amperes. Presumably, and as indicated in this figure, as the field strength rises beyond 37- gausses, no current is permitted to flow through the tube so that at this point, a complete cut-off of the current is efiected.

It will be noted that the vanes 35 materially aid in conducting the magnetic field through the tube without appreciable loss at the position, i. e. within the deflection chamber, where it is used to defiect the electrons. The electrons which are deflected by the magnetic field of, course return conductively to the cathode and hence, cannot build up any deleterious electric charge on any of the conducting members within the tube.

The McArthur application Serial No. 722,917, filed April 28, 1934 and entitled Electric discharge devices, assigned to the-same assignee as the present invention, discloses and claims the adaptation of the magnetic method of controlling the initation oi the are or glow discharge to a tube in which the electrons move radially or sidewise from a cathode to a surrounding or adjacent anode.

The McArthur application Serial No. 722,919, filed April 28, 1934 and entitled Control systems for gaseous discharge device," assigned to the same assignee as the present invention, discloses and claims the feature of introducing a phase relation between the current energizing the are or glow discharge device and the magnetomotive force of the magnetic control apparatus.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination, an electron discharge device comprising an envelope containing a source of electrons, an electron-receiving member, and an electrode mounted therebetween, a source of electromotive force connected between the electron source and the electron-receiving member, said electrode having such a configuration and being electrically charged so as to provide an electrostatic field of low intensity in the region of said electron source, an ionizable medium in said envelope at a pressure under operating conditions suflicient to support an arc-like discharge, means for producing a magnetic field which intercepts the direction of said discharge in the region of said low intensity electrostatic field said magnetic field having a maximum strength suiilcient to prevent initiation of the discharge, means for controlling the strength of the magnetic field whereby initiation of the discharge and the current flowing through the device are controlled.

2. In combination, an electron discharge device comprising an envelope containing a cathode, an anode and an electrode mounted therebetween, a source of electromotlve force connected between the cathode and anode, said electrode having such a configuration and being adapted to be charged negatively with respect to the cathode so as to provide an electrostatic field of low intensity in the region of said cathode, an ionizable medium in said envelope at a pressure under operating conditions suflcient to support an arc-like discharge, means for producing a magnetic field which intercepts the direction of said discharge, said magnetic field being suificiently strong to prevent initiation of the discharge when said electrode is charged negatively, and means for controlling the amount oi. negative charge on said electrode whereby the initiation of the discharge is controlled.

3. In combination, an electron discharge device comprising an envelope containing a cathode, an anode and an electrode mounted therebetween, a source 01 electromotive force connected between the cathode and anode, said electrode having such a configuration and adapted to be charged negatively with respect to the cathode, so as to provide an electrostatic field of low intensity in the region of said cathode, an ionizable medium in said envelope at a pressure under operating conditions sufilcient to sup port an arc-like discharge, means for producing a magnetic field which intercepts the direction of said discharge, said magnetic field being sutficiently strong to prevent initiation of the discharge, and means for controlling the strength of the field and the amount of the negative charge on said electrode whereby the initiation of the discharge is controlled.

4. In combination, an electron discharge device comprising an envelope containing a cathode, an anode, and an electrode mounted therebetween, a source of alternating electromotive force connected between the cathode and anode, said electrode having such a configuration and being charged so as to provide an electrostatic field of low intensity in the region of said cathode, an ionizable medium in said envelope having a pressure under operating conditions sufficient to support an arc-like discharge, means mounted exteriorly of the envelope for producing a magnetic field in a direction normal to the direction of said discharge, said magnetic field having a maximum strength sufficient to prevent initiation of the discharge during positive half-cycles of the voltage impressed between the cathode and anode, and means for controlling the strength of the magnetic field whereby initiation of the discharge and the average value of the current through the device are controlled.

5. In combination, an electron discharge device comprising an envelope containing a cathode, an anode, and an electron-deflecting chamber positioned between the cathode and anode, said chamber being substantially closed except for a discharge opening, a source of electromotive force connecmd between the cathode and anode, an. ionizable medium in said envelope having a pressure under operating conditions sufiicient to support an arc-like discharge, means mounted exteriorly oi the envelope for causing the electrons in the deflection chamber to be deflected away from said opening, said means including a magnetic field of sufilcient strength to restrain current from fiowing through the opening, and means for varying the strength of the field whereby initiation of the discharge is controlled.

6. In combination, an electron discharge device comprising an envelope containing a cathode, an anode, and an electron-deflecting chamher positioned between the cathode and anode, said chamber being substantially closed except tor a discharge opening in line with the cathode and anode, a source or 'electromotive, force connected between the cathode and anode, 'an ionizable medium in said envelope having a pressure under operating conditions suflicient to support an arc-like discharge, means mounted exteriorly of the envelope for causing the electrons in the deflection chamber to be be deflected away from said opening whereby the number of electrons reaching the anode will be less than to produce sufiicient cumulative ionization to initiate a discharge within the device, said means including a magnetic field or suflicient strength to restrain current from flowing through the dill till

opening, and means for varying the strength or the field whereby initiation of the discharge is controlled.

i. In combination, an electron discharge device comprising an envelope containing a cathode, an anode and an electrode, said electrode including a cylindrical portion surrounding the cathode and anode, and a transverse portion mounted between the cathode and anode, sources oi electromotive force connected respectively be tween the cathode and anode, and between the cathode and said electrode, an ionizable medium in said envelope having a pressure under operating conditions suiilcient to support an arc-litre discharge, means mounted exteriorly of the era-- velope for producing a magnetic field in a direction normal to the direction of said discharge and in an area bounded by the cathode and the transverse portion of said electrode, said iield having a maximum strength sumcient to prevent initiation of the discharge when said electrode is biased negatively with respect to the cathode, and means for controlling the strength oi the hold whereby the initiation of the discharge and the current through the device are controlled.

d. In combination, an electron discharge device comprising an envelope containing a oathode, an anode and a third electrode, said third electrode including a cylindrical portion surrounding the cathode and anode, and a trans= verse portion mounted between the cathode and anode, sources of electromotive force connected respectively between the cathode and anode, and between the cathode and said third electrode, an ionizable medium in said envelope having a pressure under operating conditions suiilcient to support an arc-like discharge, means mounted exteriorly oi the envelope for producing a magnetic field in a direction normal to the direction of said discharge and in an area bounded by the cathode and the transverse portion of said third electrode, said field having a maximum strength scient to prevent initiation of the discharge when said third electrode is biased negatively with respect to the cathode, and means for con trolling the strength of the field and the amount of negative charge on said third electrode whereby initiation oi the discharge and the current through the device are controlled.

9. In combination, an electron discharge device comprising an envelope containing a cathode, an anode, a shield surrounding the anode and a pair oi members of magnetic material secured to opposite sides of the cathode and extending toward the envelope, said shield secured to said members whereby the shield is maintained at cathode potential, a source of electromotive force connected between the cathode and anode, anionizable medium in said envelope at required a pressure under operating conditions suflicient to support an arc-like discharge, a magnet mounted exteriorly oi the envelope and having opposite poles arranged on opposite sides 01' the envelope, in line with said members of magnetic material, the field of said magnet having a maximum strength sufiicient to prevent initiation of the discharge, and means for controlling the strength of the field whereby the initiation of the discharge and the current through the device are controlled.

10. An electron discharge device comprising an envelope containing a cathode, an anode, a cylindrical member surrounding the anode and pro vided with a restricted discharge opening, strips of magnetic material secured to opposite sides of the cathode and extending toward the envelope, said member being secured to said strips, an ionizable medium in said envelope at a pressure underv operating conditions sufflcient to support an arelike discharge.

11. In combination, an electron discharge tievice comprising an envelope containing a cathode, an anode, a cylindrical member interposed between the cathode and anode, said member being provided with a transverse bade having a discharge opening therein, strips of magnetic material secured to opposite sides oi the cathode and extending toward the envelope, said member being secured to said strips, an ionizable medium in said envelope at a pressure under operating conditions sumcient to support an arc-litre discharge, and a magnet mounted exteriorly oi the envelope and having pole pieces positioned in line with said strips.

12. In combination, an electron discharge device comprising an envelope containing a source of electrons, an anode, a hollow electrode interposed between said source and said anode and provided with a transverse web having a discharge opening therein, strips of magnetic material secured to opposite sides of said electron source and extending toward the envelope, said hollow electrode being secured to said strips, an ionicable medium in said envelope at a pressure under operating conditions sumcient to support an arelike discharge, a source oi alternating electromotive force connected between the anode and cathode, magnetic pole pieces mounted exteriorly of the envelope in line with said strips, and means for controlling the magnetic field passing through said pole pieces into said strips whereby the humher of electrons emitted by said electron source and flowing through said discharge opening is controlled and the arc is restrained during a pre-= determined portion of the positive half-cycle oi the anode voltage.

id. in the art oi controlling the initiation oi an arc discharge device, said device being energized by alternating current and comprising an envelope containing a cathode, an anode, a hollow member through which the electrons pass on their way to the anode, an ionizable medium in said envelope at a pressure under operating conditions suificient to support an arc-like discharge, the method which consists in magnetically deflecting the electrons emitted by the cathode out of their normal rectilinear paths to such an extent that the number reaching the anode is less than required to produce suificient ionization of the gaseous medium to initiate a discharge during a predetermined portion of the positive half-cycle of the anode voltage and the formation of an arc is restrained during the said portion of the anode voltage cycle.

14. In the art of controlling the initiation of an arc-like discharge in an envelope containing an electron-emitting cathode, an anode, an interposed electrode adapted to serve as a barrier for electrons which move in curved paths and providing a region wherein the electrostatic field is of relatively low intensity, and an ionizable medium at a pressure under operating conditions suflicient to support an arc-like discharge, the

method which consists in magnetically deflecting the electrons at a position between the cathode and said electrode where the electrostatic field is of relatively low intensity whereby the electrons are moved out 0! their normal rectilinear paths to such an extent that the electrons are intercepted by said interposed electrode and the formation of an arc is restrained.

' ELMER D. McAR'I'EUR. 

